Sync Your Mobile Phone with Google Calendar

GooSync is a free option that works without installing any application (over-the-air). You just set up an account, give GooSync access to your Google Calendar and configure your phone to work with GooSync.
GooSync works with mobile devices that support SyncML, and these include Blackberries, most Nokia phones, many Sony Ericsson phones and others. You can also install SyncML clients for Palm or Windows Mobile phones.
Besides synchronizing the events, GooSync tries to keep the reminders in sync. Because Google Calendar has some predefined intervals for reminders, this will not work if you define custom reminders for your phone's events.
The service's main limitations are that "you are only able to synchronize your primary Google Calendar and you are limited to a sync window of 7 days past and 30 days future of the current date," but if you pay a subscription these limitations are removed.